A method that achieves focusing by moving a lens or lenses is widely employed to implement a focusing system for varying the focal length or focus position of an optical system. However, this method requires the use of a lens driving mechanism, and therefore has the drawbacks that the focusing system is complex, and that a relatively large amount of power is required to drive the lens driving motor. A further drawback is that impact resistance is generally low. In view of this, as a focusing system that does not require the use of a lens driving mechanism, there is proposed a system that achieves focusing by varying the refractive index of a liquid crystal lens (for example, refer to Patent Document 1).
On the other hand, a contour detection method, which extracts information corresponding to an image out-of-focus condition directly from the captured video image, and controls the lens to minimize the out-of-focus condition by using a hill climbing method, is known for implementing an autofocus (automatic focusing) system for a video camera. Various kinds of autofocus apparatus using such a hill climbing control method are proposed (for example, refer to Patent Documents 2, 3, 4, and 5).
Patent Document 1: Japanese Patent No. 3047082 (Pages 3 to 5, FIGS. 1 to 4)
Patent Document 2: Japanese Utility Patent Publication No. H02-44248 (Pages 4 to 10, FIGS. 7 to 11)
Patent Document 3: Japanese Patent No. 2742741 (Pages 1 and 2, FIGS. 5 to 7)
Patent Document 4: Japanese Examined Patent Publication No. H01-15188 (Pages 1 to 3, FIGS. 1 to 5)
Patent Document 5: Japanese Examined Patent Publication No. H02-11068 (Pages 3 to 5, FIGS. 1 to 3)